Technical Field
The technique disclosed herein relates to a gas shielded metal arc welding method for use in, for example, butt welding of steel plates.
Description of Related Art
In recent years, an increase in traffic volume or an increase in vehicle weight has caused damage to already-existing bridges, and the already-existing bridges, therefore, have been repaired or reinforced. To address such an increase in traffic volume or such an increase in vehicle weight, the functionality of the bridges has been enhanced. For example, the width of each bridge (the number of lanes) has been increased, or such bridges have been upgraded from second-class bridges (accommodating cars that can carry a load of 14 tons) to first-class bridges (accommodating cars that can carry a load of 20 tons).
Typically, a bridge cannot be carried to a factory, and various works described above are, therefore, executed at the site of the bridge. If the bridge is closed to execute the works, a need exists for executing the works only during the night during which the traffic volume is relatively low or for allowing vehicles to run in at least one of lanes to reduce the influence of the closure of the bridge on communities or the economy. As a result, problems, such as lack of time for executing the works, the securement of the safety of operators, or the influence of the works on residents living in the vicinity of the bridge (e.g., noise), have become more significant.
To address such problems, there has been an increasing demand for executing the works while the bridge is in service without closing the bridge. When the works are executed while the bridge is in service as described above, examples of a process for bonding steel plates include weld connecting and high strength bolted connection, and conventionally, high strength bolted connection has been principally used for bridges. The reason for this has been considered to be that since vehicles running on a bridge in service vibrate the bridge, and displace bevels, welding under such conditions prevents the reliability of weld joints from being secured.
FIG. 7 illustrates how a vehicle runs on a steel plate deck girder bridge. A portion of the bridge illustrated in FIG. 7 includes a left lane block 11 and right lane blocks 12 and 13. The left lane block 11 is weld-bonded to each of the right lane blocks 12 and 13, and the right lane blocks 12 and 13 are weld-bonded to each other. Here, when a vehicle 15 runs on the left lane block 11, welded joints are vibrated, and bevels are displaced.
However, even in the case of high strength bolted connection, when the works are executed under a load and vibration conditions, the mechanical performance of a joint is not always clear. Comparison between weld connecting and high strength bolted connection shows that as seen from these bonding processes, weld connecting provides a simpler structure, a higher degree of flexibility, and better bonding efficiency. For this reason, if in-service welding can be performed, weld connecting is significantly advantageous. Thus, for example, PATENT DOCUMENT 1 proposes an in-service welding method.
PATENT DOCUMENT 1, as referenced herein, refers to Japanese Unexamined Patent Publication No. H06-170539.